Steerable drilling systems are well known and take a range of forms. In one arrangement, a rotatable drill bit is mounted upon a housing at an angle to the axis of the adjacent part of the borehole. By controlling the angular position of the housing, and hence the orientation of the drill bit, specifically the axis of rotation thereof, the drilling direction can be controlled. Another form of steerable drilling system includes a drill bit secured to a bias unit, the bias unit having a plurality of bias pads associated therewith, each of which is movable between a retracted position and an extended position. Each bias pad, when in its extended position, bears against the wall of the borehole resulting in the application of a lateral reaction force to the bias unit, and hence to the drill bit. By appropriate control over the timing of the movement of the bias pads relative to rotation of the bias unit, the system can be controlled so as to urge the drill bit in a desired direction, hence enabling drilling of the borehole in a desired direction or along a desired path.
GB2423102 describes an arrangement in which a drill bit and a bias unit are formed integrally with one another, the bias unit having provided thereon a series of pivotable bias pads, each of which carries a series of cutting elements.
It is desirable to be able to provide a system which is of reduced axial length and in which relatively little power is consumed in the operation of the system.